Make it clear that client do not use __init__

The clients never get passed useful values to their `__init__` methods.
Rather the real values are passed through later using the `init_app`
method.

So it should be an error if the client is relying on the values that
get passed to it’s init method. Easiest way to ensure this is by making
the `__init__` method not expect any arguments and passing fake values
to the `Super` call.

app/notify_client/service_api_client.py

@@ -9,9 +9,7 @@ class ServiceAPIClient(NotificationsAPIClient):
     # Fudge assert in the super __init__ so
     # we can set those variables later.
     def __init__(self):
-        super(ServiceAPIClient, self).__init__("api_url",
-                                               "client",
-                                               "api_key")
+        super(self.__class__, self).__init__("a", "b", "c")
 
     def init_app(self, application):
         self.base_url = application.config['API_HOST_NAME']